High strength magnesium-lithium base alloys



United States Patent the Army No Drawing. Filed July 20, 1%2, Ser. No.211,456

8 tjlaims. (Cl. 75168) (Granted under Title 35, US. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government for governmental purposes without the payment to meof any royalty thereon.

This invention relates to magnesium-lithium base alloys and moreparticularly concerns such alloys modified by and incorporatingtherewithin minor additions of other metals for improving the mechanicaland stability properties of the alloy.

Magnesium alloys are being extensively employed in the variousstructural and mechanical arts and appear promising as light weightmaterials for rocket and missile applications. Their use in suchstructural forms, however, which are subjected to elevated temperatures,has not developed to any marked degree due to the fact that magnesiumalloys, in general, rapidly lose their strength properties and abilityto withstand plastic deformation at elevated temperatures to the extentthat they are generally unsuitable for such uses.

It is accordingly the principal object of the invention to provide amagnesium-lithium base alloy having improved mechanical and stabilityproperties in the as cast and heattreated conditions.

3,119,689 Patented Jan. 28, 1964 Other objects and advantages of theinvention will be apparent as the description of the invention proceeds.

I have found that alloys containing magnesium and lithium and suitablydetermined percentages of several or all of the four elements, aluminum,zinc, silver and silicon, in varying combinations and proportions, areparticularly favorable in the achievement of the aforementioned objects.

According to the present invention, alloys of the kind referred tocontain about 10.3 to 15 weight precent lithium and between 0.5 to 5.75weight percent of the aforementioned additive metals, the remainderbeing substantially all magnesium. The phrase substantially allmagnesium is intended to indicate a high purity magnesium with traceamounts of customary impurities, the sodium content being extremely low.The other additive elements comprising my useful alloys are similarly ofhigh purity, the total sodium content contained within the resultantalloy falling below about 0.02 weight percent.

In a preferred embodiment of my invention, the weight percentcomposition is as follows: 1.0 to 1.5 aluminum, 1.0 to 1.5 zinc, 1 to 3silver, 0.1 to 2.0 silicon, 10.5 to 15 lithium, remainder substantiallyall magnesium.

This alloy, when, by way of example, tested in the as cast condition,shows excellent ultimate tensile strength with a good refined grainsize. When heat treated at 800 F. for 4 hours, Water quenched, aged at225 F. for 6 hours and then held at 165 F. for days, the alloy exhibitedexcellent stability properties.

In the following tabulations, some more examples of alloys according tomy invention are given, it being understood that for all alloys in thetabulations, the remainder is substantially all magnesium as definedabove.

Table I Heat Treated at Heat Treated at Heat Treated at Heat Treated at800 F., 4 hrs; 800 F., 4 hrs.; 800 F., 4 hrs.; 800 F., 4. hrs; AlloyComposition, Weight Percent As Cast Water Quench- Water Quench- WaterQucncli- Water Quenched; Aged 2 hrs, ed; Agorl 6 hrs, ed; Aged 6 hrs.,cd; Aged 24 1ll'S., 225 F. 225 F. 225 F. 225 F.

Alloy No.

Ultimate Elon- Elon- Elon- Elon Elon- Li Al Ag Zn Si Tensile gation Utsgation Uts gation Uts gation Uts gation Strength, in 2, in 2, in 2, in2", in 2",

p.s.i. Percent Percent Percent Percent Percent l 15, 000 30.0 General ystable 0.5 19,750 23.0 Generally stable 1. O 0.5 36, 400 1. 0 30,000 6.0 30, 000 7.0 27, 000 12.0 1. 0 26, 300 nil 33,000 3. 0 27, 000 8.0 27,900 7.0 22,000 12.0 1. 0 0. 5 36, 700 0.5 40, 000 0.5 42, 450 1. 0 32,000 11.0 28, 600 12.0 1. 0 1. 5 35, 000 1. 0 33,100 1. 0 34,100 2. 034., 800 3. 0 28, 200 8. 0

Table II [Heat Treated at 800 F., 4 Hours; Water Quenched; Aged at 225F. for 6 Hours] Time at F. Alloy Composition, Weight Percent 0 Days 5Days 14 Days 30 Days 80 Days Alloy No.

Eloug. Elena Elong. Elong E1011; Li Al Ag Zn Si Uts in 2", Uts in 2, Utsin 2", Uts in 2", Uts in 2",

Percent Percent Percent Percent Percent Table III [As Cast; Time at 225I".]

Alloy Composition, Days Days 14 Days 30 Days 80 Days Weight PercentAlloy N0.

Elcng. Elong. Elong. Elong. Elong L1 Si Uts in 2", Uts in 2", Uts in 2",Uts in 2", Uts in 2,

percent percent percent percent percent In Table I, Alloy No. 5, whenheat treated at 800 F. for 4 hours, water quenched, and aged at 225 F.for 24 hours, yielded an ultimate tensile strength value of 28,600p.s.i. with 12% elongation in 2 inches as compared to the less stableAlloy Nos. 1 and 2 which contained no silver or silicon. Alloy No. 4,identical with Alloy No. 5, but having no silicon content, produced aslightly more ductile material only when heat treated at 800 F. for 4hours, water quenched and aged at 225 F. for 2 and 4 hours. Its tensilestrength was lower in all cases than when silicon was employed. In eachof the alloys tested, silicon additions were made to the molten alloy inthe form of high purity silicon metal or as a master alloy of aluminum.

Table I shows the outstandingly superior stability properties of myalloys. For example, Alloy No. 5, when heat treated at 800 F. for 4hours, water quenched, and aged at 225 F. for 6 hours, and thenmaintained at 165 F. for 80 days, still yielded an ultimate tensilestrength of 26,000 p.s.i. and an elongation in 2 inches of 21%.Similarly, Alloy No. 6 yielded an ultimate tensile strength of 27,000psi. and an elongation in 2 inches of 10%. Alloy No. 1, consisting onlyof magnesium and lithium possesses an ultimate tensile strength of about15,000 p.s.i., and remains generally stable even after theaforementioned 80 day test.

Alloy No. l is considerably improved by the addition of 0.5% silicon, asevidenced by Alloy No. 2 in Table III. When these alloys were held at225 F. for 80 days, Alloy No. 2, consisting of MgLi-Si, showed anincrease of 37% in ultimate tensile strength over the Mg-Li alloy and adecrease of 25% in elongation in 2 inches.

It is apparent from the foregoing description that I have provided newand useful magnesium-lithium base alloys having high ultimate tensilestrength values even after subjection to elevated temperatures for veryprolonged periods. And even though my alloys are accompanied by somebrittleness, it is not enough to prevent their being employed for manyuseful purposes.

I claim:

1. A magnesium base alloy consisting essentially of aluminum, zinc,silver, silicon, lithium and magnesium, aluminum being present in theamount from 1.0 to 1.5 percent, zinc being present in the amount from1.0 to 1.5 percent, silver being present in the amount from 1.0 to 3.0percent, silicon being present in the amount from 0.1 to 2.0 percent,lithium being present in the amount from 10.5 to 15.0 percent, and thebalance essentially all magnesium.

2. A magnesium base alloy characterized by having an ultimate tensilestrength of at least 28,000 p.s.i. and an elongation of about 12% in 2inches after being heat treated at 800 F. for 4 hours, water quenchedand aged at 225 F. for 24 hours and consisting essentially of about 1.0%aluminum, about 3.0% silver, about 1.0% zinc, about 0.5 silicon, about14.0% lithium and the balance magnesium.

3. A magnesium base alloy characterized by having an ultimate tensilestrength of at least 42,400 p.s.i. and an elongation of about 1% in 2inches after being heat treated at 800 F. for 4 hours, water quenchedand aged at 225 F. for 6 hours and consisting essentially of about 1.0%aluminum, about 3.0% silver, about 1.0% zinc, about 0.5% silicon, about14.0% lithium and the balance magnesium.

4. A magnesium base alloy characterized by having an ultimate tensilestrength of at least about 26,000 p.s.i. and an elongation of about 21%in 2 inches after being heat treated at 800 F. for 4 hours, waterquenched, aged at 225 F. for 6 hours and held at 165 F. for days andconsisting essentially of about 1.0% aluminum, about 3.0% silver, about1.0% zinc, about 0.5% silicon, about 14.0% lithium and the balancemagnesium.

5. A magnesium base alloy characterized by having an ultimate tensilestrength of at least about 27,000 p.s.i. and an elongation of about 12%in 2 inches after being heat treated at 800 F. for 4 hours, waterquenched and aged at 225 F. for 6 hours and consisting essentially ofabout 1.0% aluminum, 1.0% zinc, 0.5% silicon, 14.0% lithium and thebalance being magnesium.

6. A magnesium base alloy characterized by having an ultimate tensilestrength of about 19,900 p.s.i. and an elongation of about 28% in 2inches when the as cast alloy is held at 225 F. for 80 days andconsisting essentially of about 14.0% lithium, 0.5% silicon and thebalance being magnesium.

7. A magnesium base alloy characterized by having an ultimate tensilestrength of about 28,600 p.s.i and an elongation of about 12% in 2inches after being heat treated at 800 F. for 4 hours, water quenchedand aged at 225 F. for 24 hours and consisting essentially of about14.0% lithium, 1.0% aluminum, 3.0% silver, 1.0% zinc, 1.5% silicon andthe balance being magnesium.

8. A magnesium base alloy characterized by having an ultimate tensilestrength of about 22,000 p.s.i. and an elongation of about 12% in 2inches after being heat treated at 800 F. for 4 hours, water-quenched,aged at 225 F. for 24 hours and consisting essentially of about 14.0%lithium, 1.0% aluminum, 3.0% silver, 1.0% zinc, and the balance beingmagnesium.

1. A MAGNESIUM BASE ALLOY CONSISTING ESSENTIALLY OF ALUMINUM, ZINC,SILVER, SILICON, LITHIUM AND MAGNESIUM, ALUMINUM BEING PRESENT IN THEAMOUNT FROM 1.0 TO 1.5 PERCENT, ZINC BEING PRESENT IN THE AMOUNT FROM1.0 TO 1.5 PERCENT, SILVER BEING PRESENT IN THE AMOUNT FROM 1.0 TO 3.0PERCENT, SILICON BEING PRESENT IN THE AMOUNT FROM 0.1 TO 2.0 PERCENT,LITHIUM BEING PRESENT IN THE AMOUNT FROM 10.5 TO 15.0 PERCENT, AND THEBALANCE ESSENTIALLY ALL MAGNESIUM.